Cell-mediated immunity (CMI) is subject to inhibition by products of the humoral response. When this happens, the peripheral functions of effector cells are lost. It is suspected that this can cause infections to become chronic or progressive, but direct evidence is lacking. A model system has been developed to study this suspected effect of immuno complexes on cell-mediated resistance to infection. It will be tested in several experimental infections, and efforts made to develop immunizing procedures which avoid the risk of blocking CMI. The related problem of restoring CMI after T cell activity has already been suppressed will also be investigated. These objectives will be pursued in relation to three infections: malaria, leprosy and murine mammary tumors.